System and method for regenerative PWM AC power conversion

ABSTRACT

A power conversion system for driving a load is provided. The power conversion system comprises a power transformer having at least one primary winding circuit and at least one secondary winding circuit, the primary winding circuit being electrically connectable to an AC power source. The system further comprises at least one power cell, each of the at least one power cell having a power cell input connected to a respective one of the at least one secondary winding circuit. Each power cell also has a single phase output connectable to the load. A power switching arrangement including at least one power switch is connected to the power cell input and a DC bus, and a switch controller is connected to the power switching arrangement and the power cell input. The power cell also has a PWM output stage having a plurality of PWM switches connected to the DC bus and the single phase output. A local modulation controller is connected to the PWM output stage. The switch controller is adapted for monitoring and controlling a DC bus voltage according to a predetermined control methodology in both a motoring mode in which power from the AC power source is supplied to the load by the at least one power cell and a regeneration mode in which power from the load is supplied to the AC power source by the at least one power cell. The power conversion system also comprises a master controller in communication with the switch controller and the local modulation controller of each of the at least one power cell. The master controller is connectable to the load to monitor power flow thereto.

[0001] The present application is a continuation-in-part of U.S.application Ser. No. 09/683,422, which derives priority from U.S.Application No. 60/258,820, both of which are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to controlling or drivingalternating current (AC) motors. More particularly, the presentinvention relates to a method and apparatus for providing regenerativecontrol of AC motors.

[0003] A wide variety of AC medium-voltage variable speed drives forinduction motors are known which use a variation of current sourcetopology with a phase-controlled silicon controlled rectifier (SCR)input stage and a 6-pulse or 12-pulse output. This topology maysometimes have the drawbacks of harmonic line currents, a variable powerfactor, and motor torque pulsations. These traits are especiallyproblematic at higher power levels typical for medium voltage motordrives. Because of these and other disadvantages of the current sourcetopology, pulse width modulated (PWM) circuits are preferred to providemotor control. Pulse width modulation is a form of modulation in whichthe value of each instantaneous sample of the modulating wave is causedto modulate the duration of a pulse. In PWM, the modulating wave mayvary the time of occurrence of the leading edge, the trailing edge, orboth edges of the pulse. The modulating frequency may be fixed orvariable.

[0004] In a PWM circuit, a reference signal may be used to generate atrain of pulses, the width of each pulse being related to theinstantaneous value of the reference signal. The pulses may be generatedby using a comparator to compare the reference signal with a carriersignal, which may be a saw tooth or triangular wave. When the referencesignal exceeds the carrier signal, the output of the comparator is high;at other times, the output of the comparator is low. The comparatoroutput does provide a train of pulses representing the reference signal.The pulses are then used to drive an electronic switching device forintermittently applying a voltage across the load.

[0005] U.S. Pat. No. 5,625,545, (hereinafter, the “'545 patent”)discloses a medium voltage PWM drive and method suitable for controllingmedium voltage AC motors, in which a multi-phase power transformerhaving multiple secondary windings provides multi-phase power to each ofa plurality of power cells. Each power cell has a single-phase outputand is controlled by a modulation controller. Because the power cellsare connected in series, the maximum output voltage for each cell may beless than the maximum line-to-line voltage.

[0006] Each power cell of the drive disclosed in the '545 patent maycontain an ACto-DC input rectifier, a smoothing filter, an output singlephase DC-to-AC converter, and a control circuit. The input rectifiercomprises a diode bridge that accepts three-phase AC input from asecondary winding circuit of the power transformer. The input rectifiertransforms three-phase AC power into DC power that may have significantripple. To ameliorate the effects of such ripple, a smoothing filtercomposed of electrolytic capacitors is connected to the DC side of theinput rectifier. The smoothing filter also connects to the outputconverter. The output converter comprises a single-phase H-bridge ofpower transistors, such as, for example, insulated gate bipolartransistors (IGBTs). Each transistor of the output converter is operableby a local modulation control circuit. Signals for controlling the localmodulation control circuit are provided by a master modulationcontroller. This allows the control of the power contribution of thepower cell to the overall power supplied to the load.

[0007] As exemplified by the drive disclosed in the '545 patent, it ispossible to produce a medium-voltage controller with low-voltage powercells by connecting multiple cells in series on each phase output line.Serial connection of cells makes multiple voltage states per phasepossible; these multiple voltage states per phase may be used to obtainimproved waveforms.

[0008] The pulse-width modulation technique of the '545 patent allowsselective control of the duration and frequency of power cell pulseoutputs. This method can use control signals, based upon interdigitatedcarrier signals, to selectively cause a switching event in a particularpower cell. Typically, switching events are sequenced such that aswitching event occurs in only one power cell at a time.

[0009] The techniques disclosed in the '545 patent are limited in thatthey provide a motor drive that cannot be used in a regenerativeoperation mode; i.e., a mode in which power flows from the motor throughthe drive to the transformer. A non-regenerative operation (or motoring)mode is operation in which power is supplied by the transformer throughthe drive to the motor. The master modulation controller in the '545patent must carefully monitor the power flow in the system to avoid anysituation in which power would flow from the motor into the drive.Failure to control the power flow would lead to an overvoltage situationdeveloping within the power cell since the power cell has no means todispose of power from the motor. The internal diode rectifier does notpermit power to flow back to the AC mains connected to the drive. Thiseffectively prevents the use of the drive in a regenerative mode.

SUMMARY OF THE INVENTION

[0010] An aspect of the present invention provides a power conversionsystem for driving a load. The power conversion system comprises a powertransformer having at least one primary winding circuit and at least onesecondary winding circuit, the primary winding circuit beingelectrically connectable to an AC power source. The system furthercomprises at least one power cell, each of the at least one power cellhaving a power cell input connected to a respective one of the at leastone secondary winding circuit. Each power cell also has a single phaseoutput connectable to the load. A power switching arrangement includingat least one power switch is connected to the power cell input and a DCbus, and a switch controller is connected to the power switchingarrangement and the power cell input. The power cell also has a PWMoutput stage having a plurality of PWM switches connected to the DC busand the single phase output. A local modulation controller is connectedto the PWM output stage. The switch controller is adapted for monitoringand controlling a DC bus voltage according to a predetermined controlmethodology in both a motoring mode in which power from the AC powersource is supplied to the load by the at least one power cell and aregeneration mode in which power from the load is supplied to the ACpower source by the at least one power cell. The power conversion systemalso comprises a master controller in communication with the switchcontroller and the local modulation controller of each of the at leastone power cell. The master controller is connectable to the load tomonitor power flow thereto.

[0011] Other objects and advantages of the invention will be apparent toone of ordinary skill in the art upon reviewing the detailed descriptionof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The features and advantages of the present invention will beunderstood more clearly upon reading the following Detailed Descriptionof exemplary embodiments in conjunction with the accompanying drawings,in which:

[0013]FIG. 1 is a diagram of a topology for a motor drive in which powercells according to embodiments of the invention may be used;

[0014]FIG. 2 is a diagram of a power cell topology according to anembodiment of the invention;

[0015]FIG. 3 is a diagram of a power cell topology according to anembodiment of the invention;

[0016]FIG. 4 is a diagram of a power cell topology according to anembodiment of the invention;

[0017]FIG. 5 is a block diagram illustrating a power cell control systemaccording to an embodiment of the invention;

[0018]FIG. 6 is a block diagram illustrating a power cell control systemaccording to an embodiment of the invention;

[0019]FIG. 7 is a block diagram illustrating a power cell control systemaccording to an embodiment of the invention; and

[0020]FIG. 8 is a block diagram illustrating a power cell control systemaccording to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention provides a multiphase AC power conversionmethodology for use in power conversion systems configured for drivingan AC motor. The power conversion methodology uses power conversioncells that may be configured to operate in both motoring andregenerative operation modes. In addition to the power cells, the powerconversion systems of the invention comprise a multiphase transformerconfigured to supply multiphase power to each power cell. Thetransformer may comprise one or more primary windings driving aplurality of secondary windings, each secondary winding being used topower a single power cell, which, in turn, provides single phase ACpower to a load such as an AC motor. Multiple power cells having thesame output phase may be connected in series to achieve the desiredvoltage level to be supplied to the load. The voltage provided by eachpower cell may be controlled using a modulation controller that isconnected to each of the power cells.

[0022] The power cells are configured with a rectifying input stagecomprising power switching arrangement having a plurality of powerswitches for controlling power flow between the motor and thetransformer The power switches control the DC bus voltage in the powercell. When the power switches are IGBTs they may also be used to controlVARs generated by the power cells in a way that is advantageous to theowner of the equipment. The power cells also include a pulse widthmodulation (PWM) output stage that controls the output to the motor whenthe power conversion system is in motoring mode. Each power cell mayalso include a smoothing capacitor disposed between the rectifying inputstage and the PWM output stage.

[0023] The power cells of the power conversion system of the inventionalso include a switch controller that may be configured according to thetype of power switches used in the power switching arrangement. Furtherthe power switching arrangement may be configured and operated accordingto a number of control methodologies, which will be described in moredetail hereafter.

[0024] With reference to the drawings, the invention will now bediscussed in more detail. FIG. 1 illustrates an exemplary embodiment ofa power conversion system 10 according to the invention. The powerconversion system 10 comprises a multi-phase AC power transformer 12that provides multiphase power to each of a plurality of power cells 20,22, 24, 26, 28, 30, 32, 34, 36. Each power cell converts the multiphaseinput power to a conditioned, single phase output, which is supplied toan AC motor 18. Embodiments of the invention may also be used to supplypower to a DC motor or any other load.

[0025] Illustratively, the power transformer 12 may include a primarywinding circuit 14 that is powered by a three-phase AC power source (notshown). The primary winding circuit 14, which may be star- ormesh-connected, may be used to energize a plurality of three-phasesecondary winding circuits 16. The secondary winding circuits 16 may bemesh-connected or star-connected to lower the supply transformer'sK-factor and to improve harmonics control. Mesh-connected windings mayinclude, for example, delta or extended delta configurations.Star-connected windings may include, for example, wye or zig zagconfigurations. Under certain circumstances, the secondary windings 16may be manipulated to advance some of the secondary windings 16 bypreselected degrees of electrical phase and to retard other secondarywindings 16 by preselected degrees of electrical phase. Some of thesecondary windings 16 may remain substantially unshifted in phase.

[0026] It will be understood by those having ordinary skill in the artthat other transformer configurations capable of providing multiphasepower to the power cells may also be used in the present invention. Itwill also be understood that single phase secondary windings may be usedin the transformer 12.

[0027] In the power conversion system 10 shown in FIG. 1, power cellsA1, A2, and A3 (ref. nos. 20, 22 and 24, respectively) each provideoutput power having phase A. Power cells B1, B2, and B3 (ref. nos. 26,28 and 30, respectively) each provide output power having phase B. Powercells C1, C2, and C3 (ref. nos. 32, 34 and 36, respectively) eachprovide output power having phase C. As shown, power cells providing thesame output phase may be connected in series on a common output line.This makes it possible to produce a medium-voltage phase line input tothe AC motor using a plurality of low-voltage power cells. Each powercell may therefore be constructed internally to low-voltage standards.For example, each power cell may have a 600-volts rating, despite itsinclusion in a medium-voltage apparatus. Serial connections also makemultiple voltage states per phase possible. These multiple voltagestates per phase may be used to obtain improved current waveforms. Insuch an embodiment, the individual power cells may be isolated fromground, and other power cells, using insulation suitable for the mediumvoltage level being used.

[0028] In the power conversion system 10 illustrated in FIG. 1, threepower cells are provided per phase output line. Due to the serialconnection between three of the power cells in each phase output line(e.g., power cells A1, A2 and A3 in the phase A output line) it ispossible to produce a maximum output voltage magnitude of about 1800 VDCabove neutral using power cells rated at 600 volts. As will be discussedin more detail hereafter, the output of each power cell may beseparately controlled to provide any voltage level below its upperlimit. Thus, the output line voltage for each phase can be separatelycontrolled to provide an output of any level between 0 and 1800 VDC.

[0029] In other embodiments, circuits using greater or fewer than threepower cells per phase may be used to satisfy the voltage requirements ofthe inductive motor load. For example, in one embodiment which can beapplied to 2300 VAC inductive motor loads, three power cells are usedfor each of the three phase output lines. However, in anotherembodiment, which may be applied to a 4160 VAC inductive motor load,five power cells may be used for each of the three phase output lines.

[0030] Individual and collective control of the power cells of the powerconversion system 10 is provided by a master controller 40, which is incommunication with each power cell. The master controller 40 monitorsthe power flow to and output of the AC motor 18. As will be discussed inmore detail hereafter, the master controller 40 controls the output ofeach power cell and may be used to control changes in operating mode ofthe power conversion system 10.

[0031] A power cell 100 according to an illustrative embodiment of thepresent invention is illustrated in FIG. 2. In a motoring mode, thepower cell 100 receives three phase AC power from a power supply 60 thatmay be the secondary windings of a power transformer such as themultiphase AC power transformer 12 of the power conversion system 10 ofFIG. 1. The three-phase AC power is received via three separate phaselines 120, 122, 124. The power cell 100 converts the three-phase ACpower into filtered DC power using a 3-phase power switching arrangement130 and a smoothing capacitor 156. The filtered DC power is thensupplied to a load 70 using a pulse width modulation (PWM) output stage160. The load 70 may be an AC motor such as the motor 14 of FIG. 1, a DCmotor or any other load that may operate in either a power receiving orpower supplying mode. In a regenerative mode, the power cell 100receives power from the load (motor) 70 and passes it back to the powersupply 60 via the power switching arrangement 130.

[0032] In the motoring mode, the power switching arrangement 130 servesas a rectifier, converting the three phase AC input from the three phaselines 120, 122, 124 into a DC output. Rectification can produce both aDC current and ripple current. Single-phase H-bridge output converterscan reflect a ripple current at twice the frequency of an AC motor beingdriven. The DC output currents of the power switching arrangement 130generally will match the DC current of the output stage 160, but theinstantaneous ripple currents generally will not match. The power cell100 may therefore include a smoothing capacitor 156 across the bus lines126, 128 to act as a current smoothing filter, to carry the differenceripple current. It will be understood by those having ordinary skill inthe art that the smoothing capacitor 156 may be a plurality or bank ofcapacitors combined in series to condition the output of the powerswitching arrangement 130. The precise capacitor values may depend uponthe power requirements of the inductive load.

[0033] The DC power, thus conditioned, can be selectively supplied tooutput lines 100 and 182 using the PWM method. Pulse-width modulationmay be effected using a bridge converter which is composed ofsemiconductor switches (hereinafter, “PWM switches”). Such PWM switchesare preferred to be power transistors as shown by transistors 162, 164,166, 168. It is also preferred that transistors 162, 164, 166, 168 beeither fully ON or fully OFF as they operate, and not significantlymodulate pulse amplitude.

[0034] The power transistors 162, 164, 166, 168 are connected in asingle-phase H-bridge configuration. To form the H-bridge configuration,the emitter of transistor 162 may be connected to the collector oftransistor 166 to form a first transistor pair. Similarly, the emitterof transistor 164 may be connected to the collector of transistor 168 toform a second transistor pair. The first and second transistor pairs areeach connected to the DC bus lines 126, 128 with the collectors oftransistors 162, 164 being connected to the positive side (bus line 126)and the emitters of transistors 166, 168 being connected to the negativeside (bus line 128).

[0035] Overvoltage protection of each of transistors 162, 164, 166, 168may be accomplished by use of anti-parallel diodes 172, 174, 176, 178.In such an arrangement, the cathodes of diodes 172, 174, 176, 178 areconnected to the collectors of transistors 162, 164, 166, 168,respectively, and the anodes of diodes 172, 174, 176, 178 are connectedto the emitters of transistors 162, 164, 166, 168, respectively. Powertransistors 162, 164, 166, 168 may be, for example, bipolar transistorsor insulated gate bipolar transistors (IGBTs). Often such transistorsinclude the anti-parallel diodes in one package.

[0036] Power, in the form of pulse-width-modulated pulses, is deliveredto a first phase output line 180 by a connection between the emitter oftransistor 174 and the collector of transistor 178. Likewise, power isdelivered to a second phase output line 182 by a connection between theemitter of transistor 162 and the collector of transistor 166.

[0037] Transistors 162, 164, 166, 168 may be controlled by a localmodulation controller 184, which receives controlling signals from amaster controller 80 configured to control multiple power cells 100. Thelocal modulation controller 184 can select either of transistor 162 or166 to be ON, and either of transistor 164 or 168 to be ON, which willpermit power to pass to a load 70 by way of the first phase output linesegment 180 or the second phase output line segment 182, respectively.

[0038] The power switching arrangement 130 includes a pair ofcontrollable switch elements 131 for each phase line 120, 122, 124.These switch elements 131 are configured in a bridge arrangement tocontrol the DC bus voltage, which is defined as the voltage between thefirst and second bus lines 126, 128. The operation of the switchelements 131 is controlled by a switch controller 110 that is connectedto the first and second bus lines 126, 128 and is capable of sensing amagnitude of the DC bus voltage. The switch controller 110 is configuredto control operation of the switch elements 131 so as to allow selectivecontrol of the DC bus voltage and the mode of operation (i.e., motoringmode or regenerative mode) of the power cell. The switch controller 110may be configured or programmed to operate according to specific controlmethodologies tailored to the type of switch elements used.

[0039] The power switching arrangement 130 may be configured to useunidirectional switches such as SCRs or bidirectional switches such asIGBTs. In accordance with one aspect of the invention, the powerswitching arrangement 130 may comprise a plurality of SCRs arranged toprovide control of the DC bus voltage in both motoring and regenerativemodes. With reference to FIG. 3, a power cell 200 receives three phaseAC power from a power supply 60 via the three separate phase lines 120,122, 124. The power cell 200 converts the three-phase AC power intofiltered DC power using a 3-phase power switching arrangement 230 and asmoothing capacitor 156. The filtered DC power is then supplied to aload 70 using a PWM output stage 160, which is substantially identicalto that described above for the power cell 100 of FIG. 2.

[0040] The power cell 200 includes a power switching arrangement 230having a first set of-six SCRs 232, 234, 236, 238, 240, 242 connected inparallel with a second set of six SCRs 244, 246, 248, 250, 252, 254, thetwo sets being connected to the first and second bus lines 126, 128,respectively. Two opposing SCRs in each set are connected in parallel toeach of the three phase lines 120, 122, 124. Thus, in the first set, twoopposing SCRs 232, 234 are connected in parallel between the first phaseline 120 and the first bus line 126, two opposing SCRs 236, 238 areconnected in parallel between the second phase line 122 and the firstbus line 126, and two opposing SCRs 240, 242 are connected in parallelbetween the third phase line 124 and the first bus line 126. Similarly,in the second set, two opposing SCRs 244, 246 are connected in parallelbetween the first phase line 120 and the second bus line 128, twoopposing SCRs 248, 250 are connected in parallel between the secondphase line 122 and the second bus line 128, and two opposing SCRs 252,254 are connected in parallel between the third phase line 124 and thesecond bus line 128.

[0041] The power switching arrangement 230 operates in a DC bus voltageregulation mode such that when the DC voltage rises beyond apredetermined threshold, the SCRs in the power switching arrangement 230are gated to reduce the DC bus voltage. The switch controller 210 of thepower cell 200 is connected to the first and second bus lines 126, 128and is capable of sensing a magnitude of the DC bus voltage. The switchcontroller 210 provides signals to gate each of the SCRs in the powerswitching arrangement 230. The switch controller 210 operates in a firstDC bus voltage regulation mode such that when the DC voltage raisesbeyond a pre-determined threshold (e.g., 1% above a predetermined DC busvoltage value), the controller 210 commands the gates of each of theSCRs such that the DC bus voltage is reduced. This allows the power cellto maintain the DC bus voltage at a desired level. In addition toproviding the capability to control the DC bus voltage, the powerswitching arrangement 230 also provides flexibility in that the SCRs maybe gated to produce any voltage below their allowable limits. Controlmethodologies that may be used to accomplish this are discussed below.

[0042] As shown, the controller 210 may also be connected to the inputphase lines 120, 122, 124 to sense the AC line voltage present in theinput phase lines 120, 122, 124, and may be suitably configured and/orprogrammed to determine the phase of the input waveform. Based on thisphase determination, the controller 210 can determine a phase advancefor gating the SCRs. In one implementation, the switch controller 210monitors the AC waveform and uses the “zero crossing” (when the ACvoltage reverses polarity) to determine a phase adjustment for firing orgating the SCRs. This input will be related to the current flow.

[0043] The power switching arrangement 230 is configured to limit andcontrol the DC bus voltage in both motoring and regenerative modes withall of the SCRs connected in one direction (forward-conducting SCRs 232,236, 240, 244, 248, 252) controlling the DC bus voltage in the motoringmode and all of the SCRs connected in the opposing direction(reverse-conducting SCRs 234, 238, 242, 246, 250, 254) controlling theDC bus voltage in the regenerative mode.

[0044] In accordance with another aspect of the invention, the powerswitching arrangement 130 of the power cell 100 of FIG. 2 may comprise aplurality of IGBTs rather than SCR pairs. With reference to FIG. 4, apower cell 600 receives three phase AC power from a power supply 60 viathe three separate phase lines 120, 122, 124. The power cell 600converts the three-phase AC power into filtered DC power using a 3-phasepower switching arrangement 630 and a smoothing capacitor 156. Thefiltered DC power is then supplied to a load 70 using a PWM output stage160, which is substantially identical to that described above for thepower cell 100 of FIG. 2.

[0045] The power cell 600 includes a power switching arrangement 630having three pairs of IGBTs connected to the first and second bus lines126, 128. A first pair of IGBTs 632, 642 are connected to the firstphase line 120, a second pair of IGBTs 634, 644 are connected to thesecond phase line 122, and a third pair of IGBTs 636, 646 are connectedto the third phase line 124.

[0046] The power switching arrangement 630 operates in a DC bus voltageregulation mode such that when the DC voltage rises beyond apredetermined threshold, the IGBTs in the power switching arrangement630 are gated to reduce the DC bus voltage. The switch controller 610 ofthe power cell 600 is connected to the first and second bus lines 126,128 and is capable of sensing a magnitude of the DC bus voltage. Theswitch controller 610 provides signals to the gates of each IGBT in thepower switching arrangement 630. The switch controller 610 operates in afirst DC bus voltage regulation mode such that when the DC voltageraises beyond a predetermined threshold (e.g., 1% above a predeterminedDC bus voltage value), the controller 610 commands controls the IGBTssuch that the DC bus voltage is reduced. This allows the power cell tomaintain the DC bus voltage at a desired level.

[0047] As shown, the controller 610 may also be connected to the inputphase lines 120, 122, 124 to sense the AC line voltage present in theinput phase lines 120, 122, 124, and may be suitably configured and/orprogrammed to determine the phase and voltage of the input waveform.Based on the phase and voltage, the controller 610 can determine theoptimum gating pattern to apply to the IGBTs. In one implementation, theswitch controller 610 uses a PLL to monitor the phase and voltage of theinput waveform. In another implementation, the switch controllerreceives phase and voltage information from the master controller 80.

[0048]FIG. 5 illustrates a block diagram of a regenerative AC powerconversion system 300 according to an aspect of the present invention.The power conversion system 300 includes a master controller 390configured for modulation control of multiple power cells 302 to providepower to an AC motor 86. The power conversion system 300 furtherincludes an input power transformer 388 that may be configured toreceive power from a multiphase AC power source (not shown) and supplyit to a plurality of power cells 302. In FIG. 5, only a single powercell 302 is illustrated. It will be understood, however, that the powerconversion system 300 may include any number of power cells 302.

[0049] The block diagram of FIG. 5 illustrates the features of aparticular control methodology that may be used in conjunction withpower cells of the present invention. The master controller 390 of thepower conversion system 300 includes a motor torque and speed controller392, a power flow limiter 394 and a drive modulation controller 396. Themaster controller 390 monitors the currents and voltages in the AC motor86. The power flow to and from the AC motor 86 is monitored by the motortorque and speed controller 392. When in motoring mode, the drivemodulation controller 396 uses modulated switch commands to control thepower transistors of the PWM output stage 360 in order to maintain thepower flow to the AC motor 86 within the predetermined limits programmedinto the power flow limiter 394.

[0050] The power cell 302 of the power conversion system 300 includes apower switching arrangement 330, a smoothing capacitor 356 and a PWMoutput stage 360, all of which may be configured according to thetopology of the power cell 100 of FIG. 2. The power switchingarrangement 330 is configured to include power switches that control theDC bus voltage in the motoring mode and in the regenerative operationmode. These switches may be SCRs as in the power cell 200 of FIG. 3,IGBTs as in the power cell 600 of FIG. 4, or other power switches ofsimilar capability. The power cell 302 includes a switch controller 310that includes a DC bus voltage controller 312 configured to monitor theDC bus voltage across the smoothing capacitor 356. The DC bus voltagecontroller 312 is wired or programmed to include a summing junction 314that subtracts the DC bus voltage value from a predetermined fixedvoltage reference value 320 to determine a bus voltage error. The fixedvoltage reference value 320 may be pre-programmed into the switchcontroller 310. The output of the summing junction 314 may be filteredusing a high pass filter (not shown) to provide a filtered bus voltageerror signal. The switch controller 310 further includes a DC busvoltage regulator 316 which receives the bus voltage error signal anddetermines if the power switches of the power switching arrangement 330should be gated to reduce or increase the DC bus voltage. Responsive toa determination that the DC bus voltage is outside predetermined limits,a signal is sent to a switch control module 318 which determines andsends gating commands to the appropriate power switches for reducing orincreasing the DC bus voltage.

[0051] When the power switches used are SCRs, the forward SCRs in thepower switching arrangement are used to control the DC bus voltage inthe power cell. This ensures that the DC Bus voltage is never too highto prevent the correct operation of the reverse SCRs in the event ofregeneration, thereby freeing the master controller 390 from a zeroregeneration limit. Accordingly, a non-zero regeneration limit may beenforced by the master controller 390. The new limit may be a functionof the capacity of the power switching arrangement to conduct current.Such limits depend on the heatsinks, conductors and packages used toconstruct the power switching arrangement.

[0052] Accordingly, the power conversion system 300 is fully operable inboth motoring and regeneration modes. In addition, the power conversionsystem 300 provides for highly rapid switching from motoring toregeneration and back.

[0053] As suggested by FIG. 5, the switch controller 310 may bephysically located on the power cell 302. It will be understood by thoseof ordinary skill in the art that the switch controller 310 mayalternatively be located in the master controller 390. In suchembodiments, the master controller would include a switch controller 310for each power cell 302 in the power control system.

[0054]FIG. 6 illustrates a block diagram of a regenerative AC powerconversion system 400 according to an aspect of the present invention.The power conversion system 400 includes a master controller 490configured for modulation control of multiple power cells 402 to providepower to an AC motor 86. The power conversion system 400 furtherincludes an input power transformer 488 that may be configured toreceive power from a multiphase AC power source (not shown) and supplyit to a plurality of power cells 402. In FIG. 6, only a single powercell 402 is illustrated. It will be understood, however, that the powerconversion system 400 may include any number of power cells 402.

[0055] The block diagram of FIG. 6 illustrates the features of aparticular control methodology that may be used in conjunction withpower cells of the present invention. The master controller 490 of thepower conversion system 400 includes a motor torque and speed controller492, a power flow limiter 494 and a drive modulation controller 496. Themaster controller 490 monitors the currents and voltages in the AC motor86. The power flow to and from the AC motor 86 is monitored by the motortorque and speed controller 492. When in motoring mode, the drivemodulation controller 496 uses modulated switch commands to control thepower transistors of the PWM output stage 460 in order to maintain thepower flow to the AC motor 86 within the predetermined limits programmedinto the power flow limiter 494.

[0056] The power cell 402 of the power conversion system 400 includes apower switching arrangement 430, a smoothing capacitor 456 and a PWMoutput stage 460, all of which may be configured according to thetopology of the power cell 100 of FIG. 2. The power switchingarrangement 430 is configured to include power switches that control theDC bus voltage in the motoring mode and in the regenerative operationmode. These switches may be SCRs as in the power cell 200 of FIG. 3,IGBTs as in the power cell 600 of FIG. 4, or other power switches ofsimilar capability. The power cell 402 includes a switch controller 410that includes a DC bus voltage controller 412 configured to monitor theDC bus voltage across the smoothing capacitor 456. The DC bus voltagecontroller 412 is wired or programmed to include a summing junction 414that subtracts the DC bus voltage value from a voltage reference value.The voltage reference value is determined by a voltage referenceselection module 420 that is in communication with the master controller490. The voltage reference selection module 420 determines whether toprovide a motoring voltage reference value 422 or a regenerative voltagereference value 424 depending on the operating mode of the powerconversion system 400. Both the predetermined motoring voltage referencevalue 422 and a regeneration voltage reference value 424 may beprogrammed or otherwise stored in the switch controller 410. Theoperating mode of the system may be signaled to the voltage referenceselection module 420 by the master controller 490. Illustratively, thismay take the form of a negative power flow signal to request a change inoperating mode in advance of the master controller 490 actuallycommanding reverse power flow.

[0057] Based on the signal from the master controller 490, the voltagereference selection module 420 selects the proper voltage reference andprovides it to the summing junction 414. The DC bus voltage is thensubtracted from the voltage reference to determine the bus voltageerror. The output of the summing junction 414 may be filtered using ahigh pass filter (not shown) to provide a filtered bus voltage errorsignal. The switch controller 410 further includes a DC bus voltageregulator 416 which receives the bus voltage error signal and determinesif the power switches of the power switching arrangement 430 should begated to reduce or increase the DC bus voltage. Responsive to adetermination that the DC bus voltage is outside predetermined limits, asignal is sent to a switch control module 418 which determines and sendsgating commands to the appropriate power switches for reducing orincreasing the DC bus voltage. When the voltage error signal has beenreduced to an acceptable level, a mode ready signal may be sent to themaster controller 490 to indicate it is safe to proceed withregenerative power flow (i.e., change the negative power flow limitvalue used by the power flow limiter 494).

[0058] When SCRs are used as the power switches in the power switchingarrangement 430, the switch control module 418 may be adapted to preventgating of the SCRs during transition from the motoring mode to theregenerative mode. Preventing SCR gating while transitioning from themotoring voltage reference value 422 to the regenerative voltagereference value 424 results in a natural reduction in voltage due to thepower being delivered to the AC motor 86 or other losses in the system.This serves to prevent an overvoltage situation that would prevent thecorrect operation of the SCRs.

[0059] As suggested by FIG. 6, the switch controller 410 may bephysically located on the power cell 402. It will be understood by thoseof ordinary skill in the art that the switch controller 410 mayalternatively be located in the master controller 490. In suchembodiments, the master controller would include a switch controller 410for each power cell 402 in the power control system.

[0060]FIG. 7 illustrates a block diagram of a regenerative AC powerconversion system 500 according to an aspect of the present invention.The power conversion system 500 includes a master controller 590configured for modulation control of multiple power cells 502 to providepower to an AC motor 86. The power conversion system 500 furtherincludes an input power transformer 588 that may be configured toreceive power from a multiphase AC power source (not shown) and supplyit to a plurality of power cells 502. In FIG. 7, only a single powercell 502 is illustrated. It will be understood, however, that the powerconversion system 500 may include any number of power cells 502.

[0061] The block diagram of FIG. 7 illustrates the features of aparticular control methodology that may be used in conjunction withpower cells of the present invention. The master controller 590 of thepower conversion system 500 includes a motor torque and speed controller592, a power flow limiter 594 and a drive modulation controller 596. Themaster controller 590 monitors the currents and voltages in the AC motor86. The power flow to and from the AC motor 86 is monitored by the motortorque and speed controller 592. When in motoring mode, the drivemodulation controller 596 uses modulated switch commands to control thepower transistors of the PWM output stage 560 in order to maintain thepower flow to the AC motor 86 within the predetermined limits programmedinto the power flow limiter 594.

[0062] The power cell 502 of the power conversion system 500 includes apower switching arrangement 530, a smoothing capacitor 556 and a PWMoutput stage 560, all of which may be configured according to thetopology of the power cell 100 of FIG. 2. The power switchingarrangement 530 is configured to include power switches that control theDC bus voltage in the motoring and regenerative operation modes. Theseswitches may be SCRs as in the power cell 200 of FIG. 3.

[0063] The control methodology used in the power conversion system 500differs significantly from that of the previous embodiments in that thecontrol of the power switches of the power switching arrangement 530 isa direct function of the operating mode of the power conversion system500. As before, the power cell 502 includes a switch controller 510 thatincludes a DC bus voltage controller 512 configured to monitor the DCbus voltage across the smoothing capacitor 556. As in the powerconversion system 200 of FIG. 3, the DC bus voltage controller 512 iswired or programmed to include a summing junction 514 that subtracts theDC bus voltage value from a predetermined fixed voltage reference value520 to determine a bus voltage error. The fixed voltage reference value520 may be pre-programmed into the switch controller 510. The output ofthe summing junction 514 may be filtered using a high pass filter (notshown) to provide a filtered bus voltage error signal. The DC busvoltage controller 512 further includes a DC bus voltage regulator 516which receives the-bus voltage error signal and determines whether thevoltage error signal is within predetermined limits. Responsive to adetermination that the DC bus voltage is outside predetermined limits, asignal is sent to a switch control module 518 having a firing commandcontroller 519, which determines optimum firing commands to send to thepower switches. A second possible firing command may be provided as afixed predetermined command value 524 programmed or otherwise stored inthe switch controller 510.

[0064] The switch control module 518 includes a firing command modeselector 526 in communication with the firing command controller 519 andthe master controller 590. The firing command mode selector 526 isconfigured to select one of the first and second firing commandsdepending on the operating mode of the conversion system 500 ascommunicated to the firing command mode selector 526 by the mastercontroller 590. The selected command is then sent to the power switchingarrangement 530. In a typical usage scenario, the firing command modeselector 526 selects the fixed command value when the power conversionsystem 500 is in the motoring mode. This has the effect of essentiallyturning the power switching arrangement 530 into a diode bridge. Whenthe master controller 590 detects that regeneration is required, itsignals the firing command mode selector 526 to switch to regenerationmode, in which the firing command mode selector 526 selects the firingcommands according to the output from the DC bus voltage controller 512.The bus voltage controller command is then used to command the powerswitches of the power switching arrangement 530 to control the DC busvoltage level in a manner similar to that of the previous embodiments. Amode ready signal may be used to signal the master controller 590 thatthe voltage error signal is within acceptable limits for changingoperation modes.

[0065] The performance of the power conversion system 500 is similar tothat of the power conversion system 400 of FIG. 6. As in the previousembodiments, the power conversion system 500 is fully operable in bothmotoring and regeneration modes. Like the power conversion system 400,the power conversion system 500 has the advantage of providingflexibility in the voltage reference used for different operating modes,which can provide a significantly better power factor during motoringoperation.

[0066] As suggested by FIG. 7, the switch controller 510 may bephysically located on the power cell 502. It will be understood by thoseof ordinary skill in the art that the switch controller 510 mayalternatively be located in the master controller 590. In suchembodiments, the master controller would include a switch controller 510for each power cell 502 in the power control system.

[0067]FIG. 8 illustrates a block diagram of a regenerative AC powerconversion system 800 according to an aspect of the present invention.This block diagram illustrates the features of another controlmethodology that may be used in conjunction with power cells of thepresent invention. This control methodology provides the capability ofcontrolling both real power and reactive power.

[0068] The power conversion system 800 includes a master controller 890configured for modulation control of multiple power cells 802 to providepower to an AC motor 86. The power conversion system 800 furtherincludes an input power transformer 888 that may be configured toreceive power from a multiphase AC power source (not shown) and supplyit to a plurality of power cells 802. In FIG. 8, only a single powercell 802 is illustrated. It will be understood, however, that the powerconversion system 800 may include any number of power cells 802.

[0069] The master controller 890 of the power conversion system 800includes a motor torque and speed controller 892, a power flow limiter894 and a drive modulation controller 896. The master controller 890monitors the currents and voltages in the AC motor 86. The power flow toand from the AC motor 86 is monitored by the motor torque and speedcontroller 892. When in motoring mode, the drive modulation controller896 uses modulated switch commands to control the power transistors ofthe PWM output stage 860 in order to maintain the power flow to the ACmotor 86 within the predetermined limits programmed into the power flowlimiter 894.

[0070] The power cell 802 of the power conversion system 800 includes apower switching arrangement 830, a smoothing capacitor 856 and a PWMoutput stage 860, all of which may be configured according to thetopology of the power cell 600 of FIG. 7. The power switches of thepower switching arrangement 830 may be IGBTs or other switches ofsimilar capability. The IGBTs of the power switching arrangement 830 areconfigured to control the DC bus voltage in the motoring mode and in theregenerative operation mode. The power cell 802 includes a switchcontroller 810 that includes a DC bus voltage and VAR controller 812configured to monitor the DC bus voltage across the smoothing capacitor856. The DC bus voltage controller 812 is wired or programmed to includea summing junction 814 that subtracts the DC bus voltage value from apredetermined fixed voltage reference value 820 to determine a busvoltage error. The fixed voltage reference value 820 may bepre-programmed into the switch controller 810. The output of the summingjunction 814 may be filtered using a high pass filter (not shown) toprovide a filtered bus voltage error signal, which is passed to a D-axiscurrent regulator 816.

[0071] The D-axis current regulator 816 controls the current that is inphase with the voltage from the input transformer 888. The switchcontroller 810 also includes a Q-axis current regulator 826 thatcontrols current that is 90 degrees out of phase with the voltage fromthe input transformer 888. Current feedback is provided to the D-axiscurrent regulator 816 and the Q-axis current regulator 826 by athree-to-two phase transformation module 828 in electrical communicationwith the input transformer 888. Output from the D-axis current regulator816 and the Q-axis current regulator 826 are passed to the switchcontrol module 818 via a two-to-three phase transformation module 817.

[0072] The power cell 802 may be operated according to a control methodin which the D-axis current regulator 816 controls the voltage of the DCbus and the Q-axis current regulator 826 controls the reactive currentsupplied to the input transformer 888. Operation of the fixed voltagereference 820 and the D-axis current regulator 816 is similar to that ofthe fixed voltage reference 320 and the DC bus voltage regulator of thesystem 300 shown in FIG. 5. Operation of the Q-axis current regulator826 can be constrained by a reserve current calculator 824 incombination with a reference limiter 822. The reactive current reference821 supplied to the reference limiter 822 may be fixed or changedperiodically by the master controller 890. Use of the reserve currentcalculator 824 to constrain the Q-axis current regulator 826 allows thepower cell to produce the maximum VARs possible given that the total ofthe D-Axis and the Q-Axis are limited by the physical capability of thepower cell 802. The reserve current calculator 824 computes theavailable current capability accounting for the D-Axis requirements,which are a function of the power demands of the AC Motor 86, and thenconstrains the Q-Axis reference current accordingly.

[0073] Producing reactive current with the Q-axis current regulator 826allows the system to supply leading or lagging VARs to the inputtransformer 888. In a system with many power cells 802, the VARs fromeach power cell 802 would be added by the input transformer 888 toprovide total VARs. VARs can be used to control the power factor atimportant points within the connected utility. Power factor control canreduce the cost the of the facility where the motor drive is installed,thus increasing the value of the motor drive to the owner of thefacility.

[0074] Embodiments of the present invention provide AC motor drives thatcombine the advantages of PWM motoring operation with the advantages ofregeneration through the use of power switches that control the DC busvoltage experienced during both modes of operation. It will, however, beunderstood by those having ordinary skill in the art that the presentinvention also encompasses power converter embodiments that make use ofa power cell configured for motoring only. These embodiments wouldrequire, for example, only forward-conducting SCRs, which would controlthe DC bus voltage of the power cells in the manner previously describedfor the motoring mode. Significantly, the present invention provides apower cell with power switches that may be controlled substantiallyindependently of the of the PWM output stage of the power cell.

[0075] Although the foregoing description includes numerous details, itwill be appreciated that these details have been included solely for thepurpose of explaining specific embodiments of the invention. Numerousand significant variations of the details provided above will be readilyapparent to persons skilled in the art which will remain within thescope and spirit of the invention, as defined by the following claimsand their legal equivalents.

What is claimed is:
 1. A power conversion system comprising: a powertransformer having at least one primary winding circuit and at least onesecondary winding circuit, the primary winding circuit beingelectrically connectable to an AC power source; at least one power cell,each of the at least one power cell having a power cell input connectedto a respective one of the at least one secondary winding circuit, asingle phase output connectable to a load, a power switching arrangementincluding at least one power switch connected to the power cell inputand a DC bus, a switch controller connected to the power switchingarrangement and the power cell input, a PWM output stage having aplurality of PWM switches connected to the DC bus and the single phaseoutput, and a local modulation controller connected to the PWM outputstage, wherein the switch controller is adapted for monitoring andcontrolling a DC bus voltage according to a predetermined controlmethodology in both a motoring mode in which power from the AC powersource is supplied to the load by the at least one power cell and aregeneration mode in which power from the load is supplied to the ACpower source by the at least one power cell; and a master controller incommunication with the switch controller and the local modulationcontroller of each of the at least one power cell, the master controllerbeing connectable to the load to monitor power flow thereto.
 2. A powerconversion system according to claim 3 wherein the master controller isconfigured to control transition of the at least one power cell from themotoring mode to the regeneration mode and from the regeneration mode tothe motoring mode using commands to at least one of the switchcontroller and the local modulation controller.
 3. A power conversionsystem according to claim 1 wherein the at least one power switchincludes a forward-conducting SCR connected to the power cell input andthe DC bus and a reverse-conducting SCR connected to the power cellinput and the DC bus.
 4. A power conversion system according to claim 1wherein the at least one power switch includes an IGBT connected to thepower cell input and the DC bus.
 5. A power conversion system accordingto claim 1 wherein the switch controller includes a DC bus voltagecontroller connected to the DC bus.
 6. A power conversion systemaccording to claim 5 wherein the DC bus voltage controller includes asumming junction configured for determining a voltage error between theDC bus voltage and a voltage reference, a DC bus voltage regulator incommunication with the summing junction, and a switch control module incommunication with the DC bus voltage regulator.
 7. A power conversionsystem according to claim 6 wherein the switch control module comprisesa firing command controller.
 8. A power conversion system according toclaim 7 wherein the firing command controller is in communication withthe power switching arrangement.
 9. A power conversion system accordingto claim 7 wherein the switch controller further includes a firingcommand mode selector in communication with the firing commandcontroller, the master controller and the power switching arrangement.10. A power conversion system according to claim 6 wherein the switchcontroller further includes a voltage reference select module incommunication with the summing junction and the master controller.
 11. Apower conversion system according to claim 1 wherein the switchcontroller includes a DC bus voltage and VAR controller connected to theDC bus.
 12. A power conversion system according to claim 1, wherein theDC bus voltage and VAR controller comprises: a summing junctionconfigured for determining a voltage error between the DC bus voltageand a voltage reference; a D-axis current regulator in communicationwith the summing junction; a reactive current reference; a Q-axiscurrent regulator in communication with the power transformer and thereactive current reference; and a switch control module in communicationwith the D-axis current regulator, the Q-axis current regulator and thepower switching arrangement.
 13. A power conversion system according toclaim 12 wherein the DC bus voltage and VAR controller further comprisesa reserve current calculator in communication with the power transformerand a reference limiter in communication with the reserve currentcalculator, the Q-axis current regulator and the reactive currentreference.
 14. A power conversion system for driving a load, the powerconversion system comprising: a multiphase power transformer having atleast one primary winding circuit and at least one secondary windingcircuit, the primary winding circuit being electrically connectable to amultiphase AC power source; at least one power cell, each of the atleast one power cell having a power cell input connected to a respectiveone of the at least one secondary winding circuit, a single phase outputconnectable to the load, a power switching arrangement including aplurality of IGBTs connected to the power cell input and a DC bus, aswitch controller connected to the power switching arrangement and thepower cell input, a PWM output stage having a plurality of PWM switchesconnected to the DC bus and the single phase output, and a localmodulation controller connected to the PWM output stage, wherein theswitch controller is adapted for monitoring and controlling a DC busvoltage according to a predetermined control methodology in both amotoring mode in which power from the AC power source is supplied to theload by the at least one power cell and a regeneration mode in whichpower from the load is supplied to the AC power source by the at leastone power cell; and a master controller in communication with the switchcontroller and the local modulation controller of each of the at leastone power cell, the master controller being connectable to the load tomonitor power flow to and from the load.
 15. A power conversion systemaccording to claim 14 wherein the master controller is configured tocontrol transition of the at least one power cell from the motoring modeto the regeneration mode and from the regeneration mode to the motoringmode using commands to at least one of the switch controller and thelocal modulation controller.
 16. A power conversion system according toclaim 14 wherein the switch controller includes a DC bus voltagecontroller connected to the DC bus.
 17. A power conversion systemaccording to claim 16 wherein the DC bus voltage controller includes asumming junction configured for determining a voltage error between theDC bus voltage and a voltage reference, a DC bus voltage regulator incommunication with the summing junction, and a switch control module incommunication with the DC bus voltage regulator.
 18. A power conversionsystem according to claim 14 wherein the switch controller includes a DCbus voltage and VAR controller connected to the DC bus.
 19. A powerconversion system according to claim 18 wherein the DC bus voltage andVAR controller comprises: a summing junction configured for determininga voltage error between the DC bus voltage and a voltage reference; aD-axis current regulator in communication with the summing junction; areactive current reference; a Q-axis current regulator in communicationwith the power transformer and the reactive current reference; and aswitch control module in communication with the D-axis currentregulator, the Q-axis current regulator and the power switchingarrangement.
 20. A power conversion system according to claim 19 whereinthe DC bus voltage and VAR controller further comprises a reservecurrent calculator in communication with the power transformer and areference limiter in communication with the reserve current calculator,the Q-axis current regulator and the reactive current reference.
 21. Apower conversion system for driving a load, the power conversion systemcomprising: a power transformer having at least one primary windingcircuit and at least one secondary winding circuit, the primary windingcircuit being electrically connectable to an AC power source; at leastone power cell, each of the at least one power cell having a power cellcircuit with a power cell input connected to a respective one of the atleast one secondary winding circuit, a DC bus and a single phase outputconnectable to the load; a power switching arrangement in the power cellcircuit of each of the at least one power cell, the power switchingarrangement including a plurality of IGBTs connected to the power cellinput and the DC bus; a switch controller associated with each of the atleast one power cell, the switch controller being connected to the powerswitching arrangement and the power cell input, the switch controllerbeing adapted for monitoring and controlling a DC bus voltage accordingto a predetermined control methodology in both a motoring mode in whichpower from the AC power source is supplied to the load by the at leastone power cell and a regeneration mode in which power from the load issupplied to the AC power source by the at least one power cell; a PWMoutput stage in the power cell circuit of each of the at least one powercell, the PWM output stage having a plurality of PWM switches connectedto the DC bus and the single phase output; a local modulation controllerassociated with each of the at least one power cell, the localmodulation controller being connected to the PWM output stage; and amaster controller in communication with the switch controller and thelocal modulation controller of each of the at least one power cell, themaster controller being connectable to the load to monitor power flowthereto.
 22. A power conversion system according to claim 21 wherein themaster controller is configured to control transition of the at leastone power cell from the motoring mode to the regeneration mode and fromthe regeneration mode to the motoring mode using commands to at leastone of the switch controller and the local modulation controller.
 23. Apower conversion system according to claim 21 wherein the switchcontroller includes a DC bus voltage controller connected to the DC bus.24. A power conversion system according to claim 23 wherein the DC busvoltage controller includes a summing junction configured fordetermining a voltage error between the DC bus voltage and a voltagereference, a DC bus voltage regulator in communication with the summingjunction, and a switch control module in communication with the DC busvoltage regulator.
 25. A power conversion system according to claim 21wherein the switch controller includes a DC bus voltage and VARcontroller connected to the DC bus.
 26. A power conversion systemaccording to claim 25 wherein the DC bus voltage and VAR controllercomprises: a summing junction configured for determining a voltage errorbetween the DC bus voltage and a voltage reference; a D-axis currentregulator in communication with the summing junction; a reactive currentreference; a Q-axis current regulator in communication with the powertransformer and the reactive current reference; and a switch controlmodule in communication with the D-axis current regulator, the Q-axiscurrent regulator and the power switching arrangement.
 27. A powerconversion system according to claim 26 wherein the DC bus voltage andVAR controller further comprises a reserve current calculator incommunication with the power transformer and a reference limiter incommunication with the reserve current calculator, the Q-axis currentregulator and the reactive current reference.
 28. A power cell for usein a power conversion system, the power cell comprising: a power cellinput connectable to a secondary winding of a power transformer; asingle phase output connectable to a load; a power switching arrangementincluding at least one power switch connected to the power cell input; aswitch controller connected to the power switching arrangement and thepower cell input; a DC bus connected to the power switching arrangement,the power switching arrangement and the switch controller being adaptedfor monitoring and controlling a DC bus voltage according to apredetermined control methodology in both a motoring mode in which powerfrom an AC power source is supplied to a load by the power cell and aregeneration mode in which power from the load is supplied to the ACpower source by the power cell; a PWM output stage having a plurality ofPWM power switches connected to the DC bus and the single phase output,the PWM power switches being configured for controlling power flow tothe single phase output; and a local modulation controller connected tothe PWM output stage, the local modulation controller being configuredfor controlling activation of the PWM power switches.
 29. A power cellaccording to claim 28 wherein the local modulation controller and theswitch controller are connectable to a master controller configured tocontrol transition of the power cell from the motoring mode to theregeneration mode and from the regeneration mode to the motoring modeusing commands to at least one of the switch controller and the localmodulation controller.
 30. A power conversion system according to claim28 wherein the at least one power switch includes a forward-conductingSCR connected to the power cell input and the DC bus and areverse-conducting SCR connected to the power cell input and the DC bus.31. A power conversion system according to claim 28 wherein the at leastone power switch includes an IGBT connected to the power cell input andthe DC bus.
 32. A power conversion system according to claim 28 whereinthe switch controller includes a DC bus voltage controller connected tothe DC bus.
 33. A power conversion system according to claim 32 whereinthe DC bus voltage controller includes a summing junction configured fordetermining a voltage error between the DC bus voltage and a voltagereference, a DC bus voltage regulator in communication with the summingjunction, and a switch control module in communication with the DC busvoltage regulator.
 34. A power conversion system according to claim 33wherein the switch control module comprises a firing command controller.35. A power conversion system according to claim 34 wherein the firingcommand controller is in communication with the power switchingarrangement.
 36. A power conversion system according to claim 34 whereinthe switch controller further includes a firing command mode selector incommunication with the firing command controller, the master controllerand the power switching arrangement.
 37. A power conversion systemaccording to claim 33 wherein the switch controller further includes avoltage reference select module in communication with the summingjunction and the master controller.
 38. A power conversion systemaccording to claim 28 wherein the switch controller includes a DC busvoltage and VAR controller connected to the DC bus.
 39. A powerconversion system according to claim 38 wherein the DC bus voltage andVAR controller comprises: a summing junction configured for determininga voltage error between the DC bus voltage and a voltage reference; aD-axis current regulator in communication with the summing junction; areactive current reference; a Q-axis current regulator in communicationwith the power transformer and the reactive current reference; and aswitch control module in communication with the D-axis currentregulator, the Q-axis current regulator and the power switchingarrangement.
 40. A power conversion system according to claim 39 whereinthe DC bus voltage and VAR controller further comprises preserve currentcalculator in communication with the power transformer and a referencelimiter in communication with the reserve current calculator, the Q-axiscurrent regulator and the reactive current reference.
 41. A power cellfor use in a power conversion system, the power cell comprising: a powercell input connectable to a secondary winding of a power transformer; asingle phase output connectable to a load; a power switching arrangementincluding a plurality of IGBTs connected to the power cell input and aDC bus, the power switching arrangement being configured for controllinga DC bus voltage; a switch controller connected to the power switchingarrangement and the power cell input, the switch controller, the switchcontroller being adapted for monitoring and controlling the DC busvoltage according to a predetermined control methodology in both amotoring mode in which power from an AC power source is supplied to aload by the power cell and a regeneration mode in which power from theload is supplied to the AC power source by the power cell; a PWM outputstage having a plurality of PWM switches connected to the DC bus and thesingle phase output, the PWM switches being configured for controllingpower flow to the single phase output; and a local modulation controllerconnected to the PWM output stage, the local modulation controller beingconfigured for controlling activation of the PWM power switches.
 42. Apower cell according to claim 41 wherein the local modulation controllerand the switch controller are connectable to a master controllerconfigured to control transition of the power cell from the motoringmode to the regeneration mode and from the regeneration mode to themotoring mode using commands to at least one of the switch controllerand the local modulation controller.
 43. A power conversion systemaccording to claim 41 wherein the switch controller includes a DC busvoltage controller connected to the DC bus.
 44. A power conversionsystem according to claim 43 wherein the DC bus voltage controllerincludes a summing junction configured for determining a voltage errorbetween the DC bus voltage and a voltage reference, a DC bus voltageregulator in communication with the summing junction, and a switchcontrol module in communication with the DC bus voltage regulator.
 45. Apower conversion system according to claim 41 wherein the switchcontroller includes a DC bus voltage and VAR controller connected to theDC bus.
 46. A power conversion system according to claim 45 wherein theDC bus voltage and VAR controller comprises: a summing junctionconfigured for determining a voltage error between the DC bus voltageand a voltage reference; a D-axis current regulator in communicationwith the summing junction; a reactive current reference; a Q-axiscurrent regulator in communication with the power transformer and thereactive current reference; and a switch control module in communicationwith the D-axis current regulator, the Q-axis current regulator and thepower switching arrangement.
 47. A power conversion system according toclaim 46 wherein the DC bus voltage and VAR controller further comprisesa reserve current calculator in communication with the power transformerand a reference limiter in communication with the reserve currentcalculator, the Q-axis current regulator and the reactive currentreference.
 48. A method of controlling a power cell circuit having apower cell input connected to an input transformer, a single phaseoutput connected to a load, a power switching arrangement having atleast one power switch connected to the power cell input, a DC busconnected to the power switching arrangement and a PWM output stagehaving a plurality of PWM switches connected to the DC bus and thesingle phase output, and a switch controller having a D-axis currentregulator and a Q-axis current regulator, the method comprising:monitoring a DC bus voltage and a power cell input current; selectivelycontrolling the DC bus voltage using the D-axis current regulator andthe power switching arrangement; selectively controlling a reactivecurrent to the input transformer using the Q-axis current regulator andthe power switching arrangement; in a motoring mode, applying power tothe load by selectively activating at least one of the PWM powerswitches to allow current to flow through the single phase output to theload; and in a regenerative operation mode, receiving power from theload and supplying it to the input transformer.
 49. A method accordingto claim 48, wherein the at least one power switch includes an IGBT.